The more bottles you use, the higher the likelihood you will be successful.

In general, the procedure is to pour off the beer (into a glass of course), swirl the dregs at the bottom and pour them into starter wort. You should go for a few bottles worth of dregs into a 500ml starter. When the 500ml starter is fermented out, you should step it up to a 1000ml or larger starter.

The key for this whole process is sanitation. It is very easy to accidentally upscale a whole batch of contaminating bacteria instead of the yeast-of-interest.